This invention generally relates to digital systems that use a fixed number of signal lines for communicating addresses to a digital memory element, or other digital storage device having a plurality of addressable memory locations, and more particularly to an apparatus and method that increases the number of available addresses capable of being used to address the memory element.
Recent electronic advances, particularly in the digital arts, have witnessed a proliferation of a wide variety of digital systems, from large scale systems incorporating a number of processing units to consumer goods incorporating microprocessors. On the consumer side, for example, the TV game industry has for some time been producing video games for home use that incorporate microprocessors to maintain and control game play operation.
One form of such a video game currently enjoying substantial popularity today includes a console unit containing the microprocessor and other electronic circuitry that receives player input information from player manipulated elements (i.e., paddles, joysticks, and the like) and generates electronic signals that are used to drive a TV display unit. The game console is provided with a receptacle that removably receives an inexpensive cartridge. The cartridge contains an electronic microcircuit, including a read-only-memory (ROM) that stores the program of the video game to be played. With a plurality of such interchangeable cartridges, a player can program the microprocessor of the video game to execute any one of a large selection of video games.
One of the potential problems with any digital system and one which has specifically developed in the video game industry, resides in the limit of the addressable memory space of the system, i.e., the number of individually addressable memory locations which can be uniquely addressed by the processor unit. This limit is related to the number of signal lines used to make up the address bus that conducts address signals to the memory space. For example, the video game type referred to above couples a portion of the system's address bus, consisting of 12 signal lines, via appropriate wiring and a connector plug to the ROM of the microcircuit contained in the cartridge. This provides for a maximum of 2.sup.12 or 4,096 uniquely addressable ROM memory locations for containing the program instructions used by the microprocessor to define the video game. As experience is gained, and programming technique improves, it has become desirable to increase the number of addressable memory locations in individual cartridges. However, conventional addressing techniques are limited by the number of address signal lines available at the game console/cartridge connector.
Accordingly, it is desirable to increase the number of addressable memory locations without changing the number of address signal lines in the current connector.